The present invention relates to a multi-cylinder gasoline engine including an engine body having a plurality of cylinders, an exhaust manifold through which exhaust gas discharged from each cylinder of the engine body passes, and a controller for controlling various components contained in the engine body and the exhaust manifold.
Conventionally, in the field of gasoline engines, a spark-ignition combustion has been generally adopted, in which mixture gas is forcibly combusted by a spark-ignition of an ignition plug; however, instead of such a spark-ignition combustion, an application of a so called compressed self-ignition combustion to gasoline engine has been under study. Compressed self-ignition combustion indicates a combustion in which mixture gas self-ignites to combust under an environment at a high temperature and pressure created by the compression of a piston. Compressed self-ignition combustion includes multiple combustions in which mixture gas in cylinders simultaneously self-ignites to combust, and a combustion period is shorter than a spark-ignition combustion in which the combustion gradually spreads by flame propagation. Therefore, it has been said that a high thermal efficiency can be obtained by compressed self-ignition combustion. Note that, hereinafter, spark-ignition combustion is simply referred to as “the SI combustion” and compressed self-ignition combustion is simply referred to as “the CI combustion.”
For example, JP2009-091994A and U.S. Pat. No. 7,290,524B2 (2007-132319A) disclose gasoline engines applied with the CI combustion.
In JP2009-091994A, an art is disclosed in which a combustion mode is switched according to an engine load, for example, the CI combustion is performed within a low engine load range and the SI combustion is performed within a high engine load range.
In U.S. Pat. No. 7,290,524B2 (2007-132319A), an art is disclosed in which in the CI combustion operation, an exhaust valve is not only opened on exhaust stroke but also on intake stroke, and high-temperature exhaust gas (burnt gas) which is discharged once, flows back into a cylinder from an exhaust port, so that an in-cylinder temperature increases and a self-ignition of mixture gas is stimulated. Note that, hereinafter, the flowback operation of exhaust gas as described in U.S. Pat. No. 7,290,524B2 (2007-132319A) is referred to as “the internal EGR” (Internal Exhaust Gas Recirculation).
Here, when the internal EGR is performed to stimulate the self-ignition of the mixture gas as U.S. Pat. No. 7,290,524B2 (2007-132319A), and especially within an extremely low engine load range where a fuel injection amount is small (and thus ignitability easily degrades), it is required to input a large amount of high-temperature exhaust gas into the cylinder by the internal EGR. However, a sufficient amount of exhaust gas cannot be inputted into the cylinder by only opening the exhaust valve on intake stroke as U.S. Pat. No. 7,290,524B2 (2007-132319A), and it may cause an insufficient increase in temperature inside the cylinder.
Moreover, in surely achieving the CI combustion within the low engine load range, it is effective to increase a compression ratio of the engine; however, when the compression ratio of the engine increases, a problem arises that an abnormal combustion, such as knocking, easily occurs within the high engine load range where the fuel injection amount is increased.
The present invention is made in view of the above situations and provides a multi-cylinder gasoline engine that can achieve both securing ignitability when performing CI combustion within a low engine load range and preventing an abnormal combustion when performing SI combustion within a high engine load range.